Circuit-Switching and Packet-Switching
Broadly speaking, communications networks transmit data according to either or both of two modes of transmission, which are usually referred to as circuit-switching and packet-switching. Some networks (and the nodes of which they are comprised) are configured to perform just one of these transmission modes; other networks may be configured to perform both, either by virtue of having nodes capable of performing both modes of transmission or by including some nodes capable of performing each mode of transmission.
An example of a circuit-switching network is the traditional telephone network, in which a telephone line (i.e. a circuit) is physically connected (i.e. switched) to another for the duration of a call. In the early days of telephony, the switching of circuits was done by human operators physically inserting connectors into sockets in order to connect lines together. Later, the switching of connections was done automatically using electromechanical switches controlled by the number dialed, and then by solid state electronics, but the result was essentially the same—a dedicated physical electrical circuit was created between two endpoints for the duration of a call. This is true even where the end-to-end path between the two end-points is made up of multiple hops on links between individual nodes, but in such cases, a circuit selection and set-up process may be required before any data is sent, in order to find the best or an appropriate series of links between the end-points that are capable of carrying the data—once a circuit has been set up, all of the data travels along the same series of reserved links.
Circuit-switching was and still is suitable for many types of real-time communication such as voice or video calls because once a circuit has been set up, it can provide guaranteed dedicated bandwidth (i.e. capacity) until the circuit is taken down again. Such a transmission mode is not always suitable for the transmission of computer data, however, in which data transmission often happens in short bursts. Dedicated bandwidth such as that provided by circuit-switching is likely to be wasted during quiet periods between such bursts, which may be of varying and unpredictable size, and may happen at unpredictable times. For such “bursty” transmission of data, packet-switching is often a more suitable transmission.
Packet-switching is a group of digital network communication techniques in which data to be transmitted is grouped into suitably-sized units, called packets, which are transmitted via a medium that may be shared by multiple simultaneous communication sessions or flows.
Generally, packets are composed of a header and a payload. Information in the header is used by networking hardware (i.e. routers or other types of nodes) to direct the packet to its intended destination, at which the payload may be extracted and used.
With packet switching a single physical link between two points can be used to carry data for different users at different times, thereby avoiding the potential waste of capacity that would result if bursty traffic were being sent on a reserved link. Also by splitting the data up into small chunks (i.e. packets) and routing or switching each one separately at each intermediate node, fault resilience can be built in as each packet can take a different route, thereby by-passing problems before the data is re-assembled if/when the chunks reach the intended receiver. The packets making up a flow (where the term flow is generally used to denote a stream of packets produced by a given application and transmitted from a particular sender towards a particular destination with a particular QoS requirement) may all traverse the same route (i.e. the series of routers or intermediate nodes between the sender and the destination), but this need not be the case, and if network conditions at particular nodes, on particular links, or overall are such that packets of the flow are better traversing the network by a number of different routes, the intermediate nodes on those routes will obtain the necessary information from the headers of the packets they receive to enable them to forward them on appropriately via those routes such that they should eventually reach and be reassembled at the intended destination. Packet-switching can therefore increase efficiency and robustness in networks, and can enable many different applications to operate within the same network.
Connection-Oriented and Connectionless Communication
As will now be explained, packet-switching technologies can themselves be split into two types of packet-switching technologies referred to as “Connection-Oriented” and “Connectionless”, meaning that resource sharing of a network can generally be regarded as being managed according to one of three main types of technique, with the following characteristics:
(i) Connection-Oriented Circuit-Switched (CO-CS), e.g. PSTN, SONET/SDH, WDM:                A path with reserved resources is established before any data can be sent        A path can be identified through e.g. wavelength, signal code or timeslot        Resource allocation is guaranteed, even if no data is being sent        Data needs only minimal labelling        
(ii) Connectionless Packet-Switched (CL-PS), e.g. Internet Protocol (IP), SS7:                Each packet carries complete information about the intended destination (and generally about the source as well) such that the route (or the next hop thereof, at least) may be determined only once the packet has arrived at a switching point.        This gives maximum flexibility enabling resources to be fully utilised.        Routing around failures is possible.        
(iii) Connection-Oriented Packet-Switched (CO-PS), e.g. Multiprotocol Label Switching (MPLS):                A fixed path is established before data is sent.        Short labels can be used.        The order of packets is maintained through transmission.        Unlike the CO-CS case, any time-sharing of the physical path is irregular in nature.        
Of these three, it will be noted that two of them (CO-CS and CO-PS) involve the establishment of an end-to-end path before data is sent, whereas the other (CL-PS) does not.
Energy Considerations:
Energy usage in a modern data communications network such as a packet network is generally dominated by the energy consumption of its routers or switches. The energy use of routers generally depends on the nature of the traffic and type of resource management that is used.
High speed routers generally have significant energy consumption nowadays because they function at least primarily as packet switches. This means that they need to process every packet they receive. Each packet needs to carry full addressing information in its header, and at every router, the header needs to be processed, generally for error handling, hop count, address look-up and route determination processes. Also, each packet is typically buffered during processing and while awaiting onwards transmission, so queuing control needs to be provided. All these overheads were historically justified because of the statistical gains that were achieved because computer traffic tended to consist (at least mainly) of delay-insensitive data messages or short flows with long gaps between them. However, nowadays, transmission is cheap and traffic is predominantly long-hold high-bandwidth video with increasing levels of delay-sensitive traffic such as low bandwidth voice and high bandwidth cloud gaming. This means that a traditional packet network is unlikely to be the best solution for all traffic. Traffic has a very diverse nature, from high-bandwidth, constant bit-rate video services to highly irregular short messages, for example, as associated with the “Internet of Things”.
The following table (Table 1) compares the three main network resource-management or resource-sharing systems from the perspective of energy use.
TABLE 1Energy Usage of Main Network Modes of OperationModeControl CostSwitching CostTransmission CostCO-CSOne-off costs to establishLowest because no bufferingLowest when there is highthe path, so ideal forrequired.utilisation from predictablelong-hold sessions. NoSwitching operation attraffic, e.g. streamed.on-going overheadtimescales of circuitCL-PSPer-packet processingHighest, to accommodateUtilisation is typically underand per-packet overhead.different packet sizes. Large50% to manage Quality ofIdeal for single messagesbuffers requiredService (QoS), increasingtransmission costsCO-PSNeed to establish pathLess processing than the CL-If admission control is used,and label each packetPS case to forward a packethigh utilisations may beand typically smaller buffersachievable
Most networks support alternative traffic modes as overlays. Thus an IP CL-PS network may be overlaid onto the CO-CS circuit switched wavelength-division multiplexing (WDM) network and a connection oriented flow (CO-PS) may be supported over IP through use of Transmission Control Protocol (TCP). The disadvantages of this approach include that this is less flexible in response to different traffic types, and energy consumption would be higher than a parallel-mode network as processing for each layer must take place.
Looking now at specific prior techniques and proposals, a paper entitled “A Radical New Router” by Lawrence G. Roberts posted online in July 2009 on the IEEE Spectrum website: http://spectrum.ieee.org/computing/netorks/a-radical-new-router/0 argues that network routers are too slow, costly, and power hungry, and proposes a way to fix this based on the idea of reducing the need for complex header processing at each router by doing one full look-up per flow and storing the results in a cheap-to-access hash table.
Circuit and packet network modes ran in parallel within some early 3G networks, a summary of which is available at http://en.wikipedia.org/wiki/UMTS_channels, and which were governed by various 3G standards available online and elsewhere. One channel (the common packet channel, CPCH) was left open for terminals to use for intermittent packet data, whilst the remaining capacity was to be used for circuit based transmission only.
United States application US2003039237 (Forslow) relates to mobile communications, and to different services and features that may be employed to establish and enhance communications between a mobile station in a mobile communications network and an external network entity.
United States application US2015071282 (Anders et al) relates to techniques and mechanisms for performing circuit-switched and packet-switched routing for network communication.
U.S. Pat. No. 6,538,989 (Carter et al) relates to packet networks, and to an approach which provides both bounded-delay and best-effort operation in a packet network node. The bounded-delay mode is capable of providing a firm end-to-end delay bound. Network nodes are provided with dual packet buffers associated with bounded-delay and best-effort classes of service respectively. Appropriate dimensioning, if necessary enforced through connection admission control (CAC) methods, may ensure that packets admitted to the bounded-delay buffer are provided the firm delay bound. CAC methods are described which are applicable for packet flows as small as a single packet.
The replacement of circuit-based transmission by packet-switching has however been a cornerstone of modern data networks. When the Internet started to develop, data files were generally of relatively modest size, and processing was both cheap and quickly becoming much faster, whilst transmission was expensive, meaning that packet-switching was the natural and most efficient solution for moving data.
Nowadays, however, streaming of large volumes of media content (e.g. audio or video content) between one source and one target over a period of time represents an increasing proportion of all data traffic. Simultaneously, improvements to speed and efficiency in processing technology are slowing down. This means that the conditions could be such that the establishment of a circuit between the source and the destination may once again become more efficient than packet-switching. This is because, once established, such an end-to-end circuit is more resource-efficient (i.e. the main cost is paid upfront, when the circuit is created). Correspondingly, the conditions could be such that transmission between the source and the destination using a connection-oriented packet-switched (i.e. CO-PS) transmission mode may be more efficient than using a connectionless packet-switched (i.e. CL-PS) transmission mode. Similarly, this may be because, once a path has been established that will be suitable for connection-oriented transmission, using such an established path is generally more resource-efficient than using a connectionless transmission mode requiring per-packet processing and per-packet overhead (i.e. similarly, a main cost is paid upfront when the path is established).
For some types of traffic, such as that composed of short messages or intermittent data, however, the overheads of circuit or path establishment may prove a significant energy cost.
Therefore, to allow a network to offer the most energy-efficient options for transmitting different types of data in different scenarios, it should be capable of providing circuit-switched and packet-switched transmission modes as parallel services within the same infrastructure and/or of providing connection-oriented and connectionless transmission modes as parallel services within the same infrastructure, and of being able to ascertain which option should be chosen in individual cases.